Efficient optical multiplexers and demultiplexers suitable for long haul and local area communications networks can be realized in integrated form. See, for example, U.S. Pat. No. 5,002,350. These efficient multiplexers and demultiplexers comprise a plurality of essentially uncoupled transmissive waveguides which effectively provide unequal path lengths between a group of input waveguides and a group of output wave guides. The unequal length waveguides essentially act as a grating whose order is determined by the difference in path length between the adjacent waveguides.
The size of multiplexer/demultiplexer devices using such transmissive waveguides to perform the function of an optical grating tends to be large. The size of these transmissive devices may, in fact, be too large to be practical, especially when the number of channels handled by the device is large, the required level of crosstalk between channels is very low, and the channel width is small. This large size is due, in part, to the long bends needed in the grating to produce the required path length differences in the grating. To maintain low levels of cross-talk between adjacent ports of the device, the spacing between waveguides must be relatively large--for example, more than twice the width of each waveguide--which requires a further increase in the size of the device. Large size is disadvantageous in itself because too much precious space is used and miniaturized devices for a communication system become harder to achieve. Also, in large integrated devices, unavoidable compositional variations in the materials making up those devices become more noticeable and thus have an increasingly deleterious effect on performance in larger devices. In the case of the multiplexing and demultiplexing device described above, compositional variations cause variations in the propagation constant of the waveguides. Changes in the propagation constant will cause phase errors in the optical signals flowing in the waveguides. Such phase errors will noticeably increase the cross-talk between channels in a large size multiplexing and demultiplexing device and will seriously reduce the efficiency of the device.